This invention relates to an apparatus and method of pattern detection based on a scanning transmission electron microscope suitable for the inspection of circuit patterns formed on a semiconductor device and mask, particularly circuit patterns formed on a mask used for the X-ray exposure.
There have been used scanning electron microscopes (SEM) or scanning transmission electron microscopes (STEM) as apparatuses for detecting microscopic structures. A pattern detecting apparatus based on SEM is described in the proceeding of the 101-th conference of the 132-th committee on the application of charged particle beams to the industry in Japanese Science Promotion Council, pp. 37-148, for example.
STEM includes imaging methods of bright field imaging, dark field imaging, Z-contrast method and element imaging, as described in the publication MICROBEAM ANALYSIS, pp. 199-206, for example.
The bright field imaging is designed to detect electrons that do not disperse by setting the opening angle of the detector to about 10.sup.-4 rad. In the case of an X-ray mask, electrons are prone to disperse more on the pattern than on the substrate, and therefore the pattern is detected darkly.
The dark field imaging is a method for detecting only electrons dispersed on an object under test. Because of the detection of electrons dispersed on a pattern, the pattern is detected brightly.
The Z-contrast method detects separately dispersing electrons and electrons that do not disperse. Thus, this method makes a contrast which depends on the atomic number from the ratio of the detected signals. The contrast is determined from the ratio of atomic numbers of the element that constitutes the pattern and the element that constitutes the substrate.
The element imaging is to measure the distribution of electron energy that is lost in an object under test. The method is capable of detecting elements having inherent values of energy loss.
Electron beam detectors used in the SEM and STEM are based on either the method of detection with a scintillator and photoelectron multiplier and the method of detection with a semiconductor detector, as described in MICROBEAM ANALYSIS, pp. 141-162.
The above-mentioned pattern detecting apparatus based on SEM involves the charge-up phenomenon when the object under test is an insulator, which in this case does not permit accurate pattern detection.
The above-mentioned imaging method of STEM provides a satisfactory contrast for a thin object under test with a thickness of about 1 .mu.m or less, in which part of the electrons are dispersed inside the object and the remaining electrons are not dispersed, while it suffers a poor contrast for a thick object under test of about 1 .mu.m or more in which most of electrons are dispersed in the object.
Conventional STEM apparatuses use objective lenses of the in-lens type, and therefore they are limited in the size of objects to be tested.
In addition, conventional electron beam detectors used in SEM and STEM have detection clock frequencies of about 4 MHz at most, and therefore the detection time cannot be reduced.